Process for producing aluminum trihydrate



Patented Apr. 17, 1951 PROCESS FOR PRODUCING ALUMINUM TRIHYDRATE w JamesR. Wall, Collinsville, 111., assignor to Aluminum Company of America,Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationJune 7, 1947, Serial No. 753,371

2 Claims. 1

btainthe seed by separating the liner par- 171C es""ntin' aluminumtrihydrate produced by auto-precipitation, as described in United StatesPatent No. 1,314,710 to Sherwin. Ordinarily, the amount of seed employedis in the neighborhood of the amount of precipitate to be produced fromthe solution, and the product obtained is in the form of particles whichare unduly large for various applications in which aluminum trihydrateis otherwise useful, for example, as a filler in the compounding ofrubber, and as a pigment.

The present invention provides a procedure for auto-precipitation ofcrystalline aluminum trihydrate from such solutions, in which seedparticles of crystalline aluminum trihydrate are employed, and'whichisparticularly adapted to the production of crystalline aluminumtrihydrate in the f9lm of extrernely fine particles. In accordance withthe invention, the seed parusly. Formal y years it has been customary h,30 mingled with amorphous aluminum hydroxide amorphous aluminumhydroxide from riikiiigaluminum t AQLILQBX? allowed to stand,..o1i1agaqueous solution ,ofan, lkali. metaLcarbinate .orv

' eieis then ,.alkalilmetal..hydgoxide,o,orrbothwln this ageing step,part of the amorphous aluminumhydrfjxide is convertedctonreryifinenparticles ofncrystalline;

aluminum ,t ihydratehwhicn thereafternseryefas I 7 seed or nucleiintheautoeprecipitation of crystalline aluminum trihydrate from analkali metal aluminate solution, such asasodiumor potassium aluminatesolution.

When the mixture of amorphous aluminum hydroxide and crystallinealuminum trihydrate produced as described above is added to the alkalimetal aluminate solution from which aluminum I trihydrate is to beprecipitated, the amorphous materialdissolves away from the very fineparticles of crystalline aluminum trihydrate, leaving 2 those particlesdispersed in the solution to act as I seed, while the dissolvedamorphous aluminum hydroxide supplies additional alumina to thesolution. The amorphous aluminum hydroxide mentioned above can beproduced from aqueous solutions of aluminum salts (such as sodiunnaluninate, aluminum sulphate and aluminum chloride) by Well known rnt'hods,in which the aluminum hydroxide is precipitated by rapid neutralizationof the aluminum salt. In carrying outtliat pic cedure in practicing thepresent invention, it is preferable to use reagents which will produce asolution of the type mentioned above, in which amorphous aluminumhydroxide precipitated can be converted to crystalline aluminumtrihydrate. For example, when the amorphous aluminum hydroxide isprecipitated by rapidly mixing carbon gigl ide gas; carbonic acid, orsodium bicarbonate solution, with an aqueous sodium aluminate solution,a slurry of precipitated amorphous aluminum hydroxide and sodiumcarbonate solution is obtained.

The size of the particles of the product obtained in theauto-precipitation of crystalline aluminum trihydrate is considerablyaffected by the sige of the seed particles. In general, the finer. theseed, the finer the product of the auto-precipitation process under anygiven set of conditions. The size of the seed obtained by the agingprocedure described above in turn depends to a great extent on theparticular conditions of time, tem- I perature, and solution alkalinityemployed in 5 partially converting amorphous aluminumhydroxide toaluminum trihydrate. The aluminum trihydrate particles increase in sizethe longer they remain in the solution, and the higher the temperatureof the solution. Likewise, the more alkaline the solution in which theconversion takes place, the larger will be the particles of aluminumtrihydrate under the same conditions of time and temperature. In anyevent, the solution should not be of such a character as to redissolvethe amorphous aluminum hydroxide, nor should the temperature employed besufficiently high for the crystalline aluminum trihydrate formed to beconverted to aluminum monohydrate.

Since through control of the conditions in which the conversion tocrystalline aluminum trihydrate is effected, the size of the particlesof aluminum trihydrate can be controlled, the present invention isparticularly adapted to the production of crystalline aluminumtrihydrate by auto-precipitation in the form of much smaller particlesthan those obtained by normal Bayer tate in undissolved form in contactwith a solu- DrOCeSS OD t result can be obt d tion of an alkali metalcompound selected from by employing in e li -p p p oc the groupconsisting of the carbonates and hyultra small particles o crystalline au droxides for sufficient time to convert only part trihydrete p c d asdescribed above from 5 of the said amorphous aluminum hydroxide toamorphous aluminum hydr crystalline aluminum trihydrate and thus pro-For example, crystalline aluminum trihydrate duce mixture of amorphousaluminum hyhaVing an average Particle Size of less than One droxide andminute particles of crystalline alumieron s produced by first rap ymixing minum trihydrate, thereafter adding the said With a Bayer ProcessSodium ehlminete Solution mixture of amorphous aluminum hydroxide and(Produced y digesting bauxite in a hot sodium aluminum trihydrate to thesaid sodium alumihydroxide-sodium carbonate solution) containt mt andagitating t lta t ix, g 88. a s p liter A1203, grems'pel wherebyaluminum trihydrate is precipitated te NaO grams D liter Nezcos, andfrom the sodium aluminate solution by auto-prewater, about twice as muchof an aqueous solucipitation.

tion containing 100.8 grams per liter of sodium 2 The process ofproducing crystalline ,1 bicarbonate, the mixtur being at a temperatureminum trihydrate from a sodium aluminate soluof about 100 F. The resu at S y, Co tion, comprising the steps of forming a slurry of ing grams pliter o gelatieus, lphe l amorphous aluminum hydroxide and an alkalialuminum hy O grams D liter Q metal carbonate solution by mixingtogether an "Na'OH, and 1 grams p lite! 0f NMCOB was alkali metalaluminate solution and a material ently agitated for 28 hours at 96 130by selected from the group composed of sodium bi- W ieh t p t Of theamorphous aluminum carbonate, carbonic acid, and carbon dioxide,hydroxide had been converted to crystalline alumaintainingthesaidqmorphous aluminum hyminum trihydrate p rt s havin an averagedroxide in undissolved form in contact with the size of less than 0.01micron (as deter by said alkali metal carbonate solution for sufficientcalculation from electron microscope diffraction time to convert m partof t aluminum patterns f h p r i The slurry was then hydroxide tocrystalline aluminum trihydrate added to a furth po on o the Same Beyerand thus produce a mixture of amorphous alup ess d u aluminate solutionmentioned 30 minum hydroxide and minute particles of crys- Ve t 2 F Inthe D D 0f 1 lltel 0f talline aluminum trihydrate, thereafter adding tslurry to liters of t Sodium illuminate the resultant mixture ofamorphous aluminum s u o T e x Wa agitated y means of a hydroxide andcrystalline aluminum trihydrate air lift for 12.8 hours while beingmaintained at to a Sodium aluminate solution and agitating about 125 to132 F., thus auto-p e p t the resultant mix, whereby crystallinealuminum tallme aluminum trlhydl'ete, Whlch was filtered trihydrate isprecipitated from the said sodium from the solution and dried. In theabove procaluminate solution by auto-precipitation ess 14.2 pounds ofvery finely divided, crystalline aluminum trihydrate were precipitatedin the JAMES R. WALL. auto-precipitation operation for each pound of 40the mixture of amorphous aluminum hydroxide REFERENCES CITED andcrystalline aluminum trihydrate employed The following references are ofrecord in the to supply the seed particles. file of this patent:

I claim:

1. The process of producing crystalline alu- UNITED STATES PATENTS minumtrihydrate from a sodium aluminate solu- Number Name Date tion,comprising the steps of precipitating armor- 1,835,271 Burke et a1. Dec.8, 1931 phous aluminum hydroxide from a solution of 2,247,624 Wall July1, 1941 an aluminum salt, maintaining the said precipi- 2,406,420 WeiserAug. '27, 1946

1. THE PROCESS FOR PRODUCING CRYSTALLINE ALUMINUM TRIHYDRATE FROM ASODIUM ALUMINATE SOLUTION, COMPRISING THE STEPS OF PRECIPITATINGARMORPHOUS ALUMINUM HYDROXIDE FROM A SOLUTION OF AN ALUMINUM SALT,MAINTAINING THE SAID PRECIPITATE IN UNDISSOLVED FORM IN CONTACT WITH ASOLUTION OF AN ALKALI METAL COMPOUND SELECTED FROM THE GROUP CONSISTINGOF THE CARBONATES AND HYDROXIDES FOR SUFFICIENT TIME TO CONVERT ONLYPART OF THE SAID AMORPHOUS ALUMINUM HYDROXIDE TO CRYSTALLINE ALUMINUMTRUHYDRATE AND THUS PRODUCE A MIXTURE OF AMORPHOUS ALUMINUM HYDROXIDEAND MINUTE PARTICLES OF CRYSTALLINE ALUMINUM TRIHYDRATE, THEREAFTERADDING THE SAID MIXTURE OF AMORPHOUS ALUMINUM HYDROXIDE AND ALUMINUMTRIHYDRATE TO THE SAID SODIUM ALUMINATE SOLUTION AND AGITATING THERESULTANT MIX, WHEREBY ALUMINUM TRIHYDRATE IS PRECIPITATED FROM THESODIUM ALUMINATE SOLUTION BY AUTO-PRECIPITATION.